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Journal of Materiomics xx (2015) 1e6www.ceramsoc.com/en/ www.journals.elsevier.com/journal-of-materiomics/

Optimization of FeSe superconductors with the high-energy ball millingaided sintering process

Shengnan Zhang a,*, Jixing Liu a,b, Jianqing Feng a, Chengshan Li a, Xiaobo Ma a,Pingxiang Zhang a

a Superconducting Materials Research Center, Northwest Institute for Nonferrous Metal Research, Xi'an, 710016, Chinab School of Materials and Metallurgical, Northeast University, Shenyang, 110016, China

Received 16 February 2015; revised 25 March 2015; accepted 10 April 2015

Available online ▪ ▪ ▪

Abstract

FeSe Superconducting bulks with high superconducting phase ratio were prepared with high energy ball milling (HEBM) aided sinteringprocess. During the HEBM process, the solid solution of FeeSe (Fe(Se)SS) can be successfully obtained, which not only assured the atomicuniformity of precursor powders, but greatly shortened the diffusion distance between Fe and Se atoms. Therefore tetragonal b-FeSe super-conducting phase can be formed directly with one step sintering process and the formation of hexagonal d-FeSe non-superconducting phase canbe avoided. The influences of Fe/Se ratio and sintering temperature on the phase transition process and the superconducting properties weresystematically studied. The change of Fe/Se ratio showed great influences on both the superconducting volume and critical temperature of finalbulks. By increasing the sintering temperature to 700 �C, FeSe bulk with higher texture, better crystallization and optimized chemicalcomposition was achieved. The FeSe superconducting bulk with the critical temperature Tc(onset) of 9.0 K was obtained with the Fe/Se ratio of1.20, HEBM time of 6 h, and sintering temperature of 700 �C.© 2015 The Authors. Production and hosting by Elsevier B.V. on behalf of The Chinese Ceramic Society. This is an open access article under theCC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/).

Keywords: FeSe; High energy ball milling; Critical temperature; Sintering temperature

1. Introduction

Since the first discovery of tetragonal FeSe as iron-basedsuperconductor with a superconducting critical temperatureTc of ~8 K in 2008 by Hsu et al. [1], it attracts many attentionsaiming at both basic research and practical applications. FeSebased superconductors also known as “11” system exhibit thesimplest lattice structures, composed of only FeeSe layers assuperconducting layer. Meanwhile, based on the similarFe2Se2 layer structures, FeSe1�xTex and FeTe1�xSx also reg-ister their distinct presence in this iron chalcogenide system,with the Tc of ~15 K. It is interesting to notice that the Tc

* Corresponding author.

E-mail address: snzhang@c-nin.com (S. Zhang).

Peer review under responsibility of The Chinese Ceramic Society.

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Materiomics (2015), http://dx.doi.org/10.1016/j.jmat.2015.04.004

http://dx.doi.org/10.1016/j.jmat.2015.04.004

2352-8478/© 2015 The Authors. Production and hosting by Elsevier B.V. on behal

BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/).

values of FeSe system can be obviously increased up to 37 Kand 55 K under high pressure [2] or by preparing unit cellFeSe ultrathin films [3e5], respectively. At the same time, theadvantages such as high upper critical field with the m0Hc2 of~47 T at 0 K [6], low cost and low toxicity of starting materialscompared to FeAs-based superconductors all suggest theirgreat potential in practical applications [7].

Recently, many investigations have been focused on thefabrications of FeSe superconductors into thin films or wiresfor practical applications. Great success is achieved with thefabrication of FeSe coated conductors, and critical currentdensity of over 105 Acm�2 is obtained under the magneticfield of 30 T [8,9]. However, the current capacity of FeSewires or tapes is much lower, only about 100 Acm�2 [10e13].The main factor which limited the current transport in wires isthe intergrain weak link, which can be attributed to both low

ductors with the high-energy ball milling aided sintering process, Journal of

f of The Chinese Ceramic Society. This is an open access article under the CC

Fig. 1. (a) XRD patterns of precursor powders with different Fe/Se ratio and

HEBM for 6 h; (b) SEM image of Fe/Se ¼ 1.0 precursor powders after HEBM

for 6 h and the EDX results of selected areas.

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filament density and the existence of secondary phases,especially the present of hexagonal non-superconductingphase, d-FeSe [14]. Therefore, the development of newfabrication techniques is much important for the applicationsof high quality FeSe wires. By developing a novel diffusiontechnique to enhance the density of FeSe tapes, the Jc valueswas increased up to 600 Acm�2 for FeSe tapes [15] and1027 Acm�2 for seven-core FeSe wires with Fe sheath [16].However, it is still very hard to avoid the appearance of sec-ondary phase during the preparation of both FeSe wires andbulks [12,15e17]. The existence of d-FeSe can not onlyreduce the superconducting volume but also decrease the Tcvalue of FeSe samples [18]. Therefore, it is very important toavoid the formation of d-FeSe phase during sintering.

Based on the study of FeeSe binary phase diagram [19,20],many FeeSe compounds can be obtained, including hexago-nal d-FeSe phase, orthorhombic Fe7Se8, and tetragonal b-FeSephase during the reaction between Fe and Se with the changeof sintering temperature [21], pressure [18], and chemicalcompositions. But only tetragonal b-FeSe phase is super-conducting. Therefore, aiming at the fabrication of super-conducting bulks with high tetragonal phase content, theinfluences of Fe/Se ratio and sintering temperature on thephase composition and superconducting properties of FeSebulks are explored based on the high energy ball milling aidedsintering process to achieve the FeSe bulks with optimizedsuperconducting properties.

2. Material and methods

High purity elemental Fe (99.9%) and Se (99%) powders(from SCR) were weighed with the designed stoichiometricmole ratio of Fe/Se ¼ 1.00, 1.10, 1.15, 1.20, 1.25, and 1.30 inglove box under the atmosphere of Ar. Then the powders weremixed and sealed under the protection of Ar in stainless steel(SS) ball milling jars as well as SS balls with a ball-to-powderweight ratio of 6:1. The high-energy ball milling machine ofSPEX-8000M was applied with the milling frequency of1720 rpm for 6 h. The powders after ball milling were coldpressed into pellets of 10 mm in diameter and 1.5 mm inthickness under a uniaxial pressure of 10 MPa. The pelletswere then sealed in evacuated quartz ampoules separately. Thesintering process was performed by heating these ampoules upto 600 and 700 �C and kept for 12 h, then slowly cooling toroom temperature with furnace.

The phase identification of both the ball milled powdersand final bulks were determined by X-ray diffractometer(XRD, Bruker D8 Advance) with Cu Ka (l ¼ 1.5406 Å) ra-diation. The phase composition was analyzed based on theReitveld refinement method performed with commercialsoftware Fullprof®. The phase distribution of bulks werecharacterized by scanning electron microscopes (SEM, JEOL-6390A). The compositional analysis was taken by Inca-X-Stream energy-dispersive X-ray spectroscopy (EDX). Andthe superconducting critical temperature was measured bymagnetization method, which was carried out by Super-conducting Quantum Interference Device (SQUID, MPMS-

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XL-7) with the applied magnetic field of 10 Oe from 4.2 to20 K.

3. Results and discussion

The influences of Fe/Se ratio on the phase composition ofHEBM powders were analyzed after the 6 h ball millingprocess. As shown in Fig. 1, with the increase of Fe/Se ratiofrom 1.00 to 1.20, only two crystallized peaks and a hump at~33� appear. The crystallized peaks can be indexed into Festructures (JCPDS #06-0696) with the peak position slightlyshifting towards higher degrees comparing with the original Fepowders as shown in the inset. In order to examine thechemical composition distribution in precursor powders, thepowders were pressed into bulk and polished. SEM imageswere taken and EDX was performed on the grains withdifferent morphology. As shown in Fig. 1 (b), it is interestingto notice that the chemical composition all over the image isuniformly distributed, with the existence of Se can be detectedeven in the center of grains. Therefore, it can be assumed thatduring the ball milling process, the Se atoms enter into the Fe

ductors with the high-energy ball milling aided sintering process, Journal of

Fig. 3. Back scattering electron images of (a) Fe/Se ¼ 1.20 and (b) Fe/

Se ¼ 1.30 bulks after sintering at 600 �C.

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matrix and FeeSe solid solution (Fe(Se)SS) forms. The humpcould be indexed into the amorphous of Fe3Se4. When the Feratio increased to 1.25, monoclinic Fe3Se4 compounds crys-talized due to the mechanical alloying effect. And with thefurther increasing of Fe content to 1.30, hexagonal Fe7Se8appeared, which implied that the Fe(Se)SS with low Se contentis not stable during the HEBM process. Besides, it can bededuced that the FeeSe binary compounds formed during ballmilling process is very sensitive to the starting Fe/Se ratio.

After the HEBM process, all the powders were cold pressedinto pellets and sintered with at 600 �C for 12 h and slowlycooled down to room temperature. The phase compositions ofthese FeeSe bulks were characterized with XRD after sin-tering. As shown in Fig. 2, the content of d-FeSe phasechanges greatly with Fe/Se ratio after the HEBM aided sin-tering process. With the original Fe/Se ratio of 1.00, the ma-jority phase is d-FeSe, and the b-FeSe content ratio is only30.2%. However, the content of d-FeSe phase decreasedrapidly with the increasing Fe/Se ratio. When the Fe/Se ratio is1.15, almost no more crystallized d-FeSe phase can bedetected, and the major secondary phase becomes to residualFe with the diffraction peak appearing at 2q ¼ 44.67�. The b-FeSe phase content of >95% is obtained at this sinteringcondition. According to the FeeSe binary phase diagram, b-FeSe phase tends to appear at higher Fe/Se ratio of over 1.0.Therefore the higher Fe/Se ratio in the precursor powders isbeneficial to the decrease of d-FeSe phase content. With thefurther increase of Fe/Se ratio from 1.15 to 1.30, although nomore d-FeSe phase appears, the residual Fe content increases,which also leads to the reduction of b-FeSe phase content inthe final bulks. Therefore the optimized Fe/Se ratio should bewithin the range of 1.10e1.20.

The back scattering electron (BSE) images of Fe/Se ¼ 1.20and 1.30 bulks were observed and shown in Fig. 3 (a) and (b),respectively. It is known that with the BSE images, phasedistribution can be recognized as contrast. Therefore, based on

Fig. 2. XRD patterns of bulks sintered at 600 �C with different Fe/Se ratio and

HEBM for 6 h.

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the analysis of images contrast combining with EDX, thedistribution of phases is analyzed. In Fig. 3 (a), although thesample is a little porous, only very small content of dark grayareas representing the residual Fe phase can be observed assecondary phase. And the overall light gray areas are identifiedas FeSe phase by EDX. While in Fig. 3 (b), due to the largeresidual Fe content, more dark gray areas are observed, whichmostly distribute at the grain boundaries of b-FeSe and willgreatly degrade the superconducting properties of these bulks.

The superconducting critical temperatures of Fe/Se ¼ 1.15,1.20 and 1.25 bulks with the heat treatment process of 600 �Cwere measured at 10 Oe with magnetization method. The zerofield cool (ZFC) curves were plotted in Fig. 4. All themagnetization values are positive due to the existence ofexcess Fe in the samples, which contributes to the ferromag-netic signal. It is noticed that the critical temperature of Fe/Se ¼ 1.15 bulk is about 4.0 K. With the increase of Fe/Se ratioto 1.20 and 1.25, two transition signal could be observed. Thelow temperature transition shows the critical temperature of

ductors with the high-energy ball milling aided sintering process, Journal of

Fig. 4. ZFC curves of Fe/Se ¼ 1.15, 1.20 and 1.25 bulks sintered at 600 �C and measured at 10 Oe from 2 K to 20 K.

Fig. 5. XRD patterns of 600 and 700 �C sintered bulks with the Fe/Se ratio of

1.20 after HEBM.

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3.5 and 3.0 K and the high temperature transition exhibit theTc of 7.4 and 7.2 K, respectively. Meanwhile, by comparingwith the change of magnetization, DM, from Tc to 2.0 K ofthese three bulks, a decrease of DM value could be noticedwith the increase of Fe/Se ratio. This could be explained withthe increasing excess Fe content in the b-FeSe matrix, which isconsistent with the previse reports in Fe(Se, Te) bulks [22,23].Therefore, it is concluded that under this sintering condition,the optimal Fe/Se ratio is 1.20, with a reasonable Tc achievedand higher superconducting phase content obtained.

The influence of sintering temperature on the phasecomposition and superconducting properties of FeSe bulkswere analyzed by sintering the Fe/Se ¼ 1.20 HEBM bulksunder 600 and 700 �C for 12 h, respectively. As shown inFig. 5, the major phase is b-FeSe regardless the sinteringtemperature, and the secondary phase of only residual Fe canbe observed. The diffraction peaks shift towards higher de-grees after the 700 �C sintering, representing the shrink oflattices. Meanwhile, the residual Fe content increased basedon the analysis of XRD patterns. Therefore, it can be deducedthat the high temperature sintering can be beneficial for therelease of excess Fe from b-FeSe matrix to form the residualFe. Besides, it is interesting to notice that the diffraction peaksof b-FeSe become stronger after the 700 �C sintering, espe-cially the (h00) and (00l ) peaks. Therefore, the crystallizationof b-FeSe phase becomes better after a higher temperaturesintering and certain texture structures are achieved.

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The superconducting critical temperatures of 700 �C sin-tered Fe/Se ¼ 1.20 bulk were measured with magnetizationmethod. The field cool and zero field cool (FC-ZFC) curvesare plotted in Fig. 6. It can be noticed that the critical tem-perature of 9.0 K is achieved. And no obvious second drop onthe ZFC curves is observed, which representing a better

ductors with the high-energy ball milling aided sintering process, Journal of

Fig. 6. FC-ZFC curves of Fe/Se ¼ 1.20 bulks sintered at 700 �C and measured

at 10 Oe from 4.2 K to 20 K.

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intergrain connections. Besides, the DM value is much largerin this sample, which represents a higher superconductingphase content. So the decrease of excess Fe in b-FeSe phaseand better crystallization after the sintering process at 700 �Ccontribute to the enhancement of superconducting properties.However, the magnetization values at normal state is higherthan zero, it implies that the sample still contains large contentof magnetic impurities. Thus further optimization studies areneeded.

4. Conclusions

The high energy ball milling aided sintering process for thefabrication of FeSe superconducting bulks was optimized bytuning the different Fe/Se ratio and sintering temperature. Theb-FeSe phase content of over 95% can be achieved when theFe/Se ratio is 1.10e1.20. The Fe/Se ratio exhibited great in-fluences on the superconducting properties of final FeSe bulks.The increase of Fe/Se ratio lead to the enhancement of criticaltemperature and the decrease of superconducting phase con-tent. Therefore, optimal Fe/Se ratio is determined to be 1.20.Optimized sintering temperature of 700 �C was obtained, atwhich the content of excess Fe decreased and the crystalli-zation behavior of b-FeSe was enhanced. Therefore, the crit-ical temperature of 9.0 K was achieved. The optimized highenergy ball milling process is very promising to be combinedwith the powder-in-tube process for the fabrication of FeSesuperconducting wires with high current capacity.

Acknowledgments

The author would like to appreciate the valuable help fromProf. Andr�e Sulpice from CRETA, CNRS on SQUID mea-surement. This research was financially supported by National“973” Project, under contract No. 2011CBA00104, NationalNatural Science Foundation of China under contract No.

Please cite this article in press as: Zhang S, et al., Optimization of FeSe supercon

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51302223, the National ITER Program of China under con-tract No. 2013GB110001, and the Program for InnovativeResearch Team in Shaanxi province under contract No.2013KCT-07.

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Please cite this article in press as: Zhang S, et al., Optimization of FeSe supercon

Materiomics (2015), http://dx.doi.org/10.1016/j.jmat.2015.04.004

Dr. Shengnan Zhang is a senior engineer in Super-

conducting materials research center, Northwest

Institute for Nonferrous Metal Research, Xi'anChina. Before joining the high temperature super-

conducting materials research group of Northwest

Institute for Nonferrous Metal Research in 2010, she

has achieved her Ph. D degree in Zhejiang University

based on the researches on thermoelectric materials.

Her recent research focuses on the fabrication of high

temperature superconducting wire (Bi-2212 and Bi-

2223) and FeSe superconductors. Based on her previ-

ous researches, she has published over 20 papers in refereed international jour-

ductors with the high-en

nals, and holds 10 Chinese patents.

ergy ball milling aided sintering process, Journal of